It is known to recovery heat of exhaust gas from an exhaust system of a vehicular engine using the principle of heat pipe and to use the recovered heat for other purposes such as for warming the engine. For example, Japanese Unexamined Patent Application Publication No. 62-268722 describes an exhaust heat recovery apparatus for heating an engine coolant using heat of an exhaust gas generated from an engine. Specifically, an evaporation unit that forms a heat pipe is disposed in an engine exhaust pipe through which the exhaust gas flows and a condensation unit that forms the heat pipe is disposed in an engine coolant circuit through which the engine coolant flows.
As another example, Japanese Unexamined Patent Application Publication No. 4-45393 describes a looped heat pipe heat exchanger. The disclosed heat exchanger includes a looped closed circulation passage filled with an internal heat-transfer fluid, an evaporation unit disposed on the circulation passage for evaporating the internal heat-transfer fluid therein by receiving external heat, and a condensation unit disposed on the circulation passage at a position higher than the evaporation unit for performing heat exchange between the evaporated internal heat-transfer fluid and an external heat-transfer fluid.
In such a heat recovery apparatus, water is suitably used as an operation fluid. In a case where the water is used as the operation fluid, a passage of the operation fluid is blocked due to freezing under a low temperature condition, such as below freezing. In such a case, an operation of the exhaust heat recovery apparatus will be stopped.
To address the above issue, Japanese Unexamined Patent Application Publication No. 6-117785 describes a method of melting the frozen operation fluid. Specifically, the temperature of a heat pipe is detected at multiple positions. When it is determined that the operation fluid is frozen, the heat pipe is heated by a heater, thereby melting the operation fluid.
Further, Japanese Unexamined Patent Application Publication No. 10-274487 describes another method of addressing the freezing of the operation fluid. Specifically, the amount of the operation fluid of a part of heat pipes is reduced smaller than that of the other heat pipes. When the operation fluid is frozen, the operation of the part of the heat pipes having smaller heat capacity is melted first in a shot time. Then, the operation fluid of the other heat pipes having larger heat capacity is gradually melted by the heat of the part of the heat pipes, which is heated first.
In an exhaust heat recovery apparatus for a vehicle, for example, it is proposed to integrate an evaporation unit with a condensation unit so as to be simple and compact, which is effective for being mounted on the vehicle. FIG. 13 shows an example of a heat recovery apparatus in which an evaporation unit J1 having heat pipes J3 and a condensation unit J2 are arranged adjacent to each other with respect to a horizontal direction. An operation fluid, which has been evaporated in the evaporation unit J1, is introduced to the condensation unit J2 through an evaporation-side communication part J71. The operation fluid, which has been condensed in the condensation unit J2, is introduced to the evaporation unit J1 through a condensation-side communication part J72.
In the heat recovery apparatus shown in FIG. 13, when the water is used as the operation fluid, the operation fluid remaining in the condensation-side communication part J72 is frozen under the low temperature condition. Thus, the condensation-side communication part J72 is blocked. If an operation of an engine is started in the condition where the condensation-side communication part J72 is blocked by the frozen operation fluid, the operation fluid is not circulated until the frozen operation fluid is melted. That is, an exhaust heat recovery is not started until the frozen operation fluid is melted. As a result, a warming up operation is likely to be retarded.
To address this issue, the method as described in the publication No. 6-117785 may be employed. However, the number of parts increases due to the heater, and further manufacturing costs increase. Moreover, a size of the heat recovery apparatus may increase. Also, it is necessary to supply the heater with electric power from a battery of the vehicle.
In a case where the amount of the operation fluid of some of heat pipes is reduced, as described in the publication No. 10-274487, the heat capacity is reduced, resulting in a decrease in heat exchange efficiency.